(a) Field of the Invention
The present invention relates to a method for manufacturing a nonmagnetic single-component developer which is used for developing latent electrostatic images in fields such as electrophotography, electrostatic recording, and electrostatic printing. More particularly, the present invention relates to a method for manufacturing a nonmagnetic single-component developer with negative chargeability which can provide clear images having an excellent image resolution and a high image density substantially without generating fog in a non-image area or background.
(b) Description of the Related Art
Various methods are used for developing latent electrostatic images. For example, in electrophotography, a uniformly charged photoreceptor composed of selenium, zinc oxide, vinyl carbazole compounds, cadmium sulfide, phthalocyanine compounds, etc. is exposed with a light image corresponding to the master drawing to extinguish the electrostatic charge on the exposed portions of the photoreceptor, thereby obtaining a latent electrostatic image on the photoreceptor. A toner composed of a binder resin, a coloring pigment, and other additives is electrostatically deposited on the latent electrostatic image to form a toner image on the photoreceptor. If necessary, the resultant toner image is transferred to an image support such as paper, and the toner thus transferred is fused by heating, softened or dissolved with a solvent, or deformed by application of pressure to be permanently fixed onto the image support.
Developments of latent electrostatic images are categorized, according to the polarities of charged toner and photoreceptor, into a normal development in which a toner and a photoreceptor are charged in opposite polarities, and a reversal development in which a toner and a photoreceptor are charged in the same polarity and a developing bias is applied to the photoreceptor.
Various methods have been used to supply a developer to a photoreceptor in the above-described development, such as a cascade method, a powder cloud method, a magnetic brush method, a jumping method, and a touch-down method. Also, electrostatic image developers are roughly categorized into two-component developers and single-component developers. The two-component developers are composed of a toner and a carrier such as iron powder, steel beads, ferrites, and glass beads having a particle size larger than that of the toner. When two-component developers are used, a latent electrostatic image is developed by the toner charged through friction with the carrier.
The single-component developers are further categorized as magnetic single-component developers and nonmagnetic single-component developers. In the magnetic single-component developers, a toner containing a magnetic substance such as tri-iron tetroxide, di-iron trioxide, and ferrite is used for forming a toner layer on a developer carrying member with the aid of magnetic force, and a latent electrostatic image is developed using the thus formed toner layer. In the nonmagnetic single-component developers, a latent electrostatic image is developed using a toner layer which is formed on a developer carrying member by contact electrification, triboelectrification, etc.
In nonmagnetic single-component development, latent electrostatic images can be developed without using a carrier or a magnetic substance. Accordingly, the development unit can be made smaller and simpler. Presently, a development apparatus utilizing a contact development is widely used. This apparatus comprises at least a toner layer forming member and a developer carrying member to provide electrostatic charges to the toner by contact electrification or triboelectrification and to form a toner layer having a uniform thickness. In this apparatus, the toner layer is contacted with a photoreceptor on which a latent electrostatic image is formed, wherein the toner is supplied onto the latent electrostatic image.
As for the toner, fine divided particles including a thermoplastic resin serving as a binder are used. A coloring pigment, a charge control agent, and other additives are dispersed in the binder resin by melt kneading, and the resulting material is finely milled. The milled particles are classified to obtain a toner comprising fine particles having a diameter of 5-30 .mu.m. Also, another type of a developer is known which is prepared by further adding other materials to the above-described toner so as to impart properties necessary for use as a developer. FIG. 3 shows a conventional process for manufacturing a developer of this type.
Examples of the thermoplastic resins include vinyl resins such as polystyrenes, acrylate polymers, styrene-acrylate copolymers and styrene-butadiene copolymers; and polyesters, epoxy resins, polyamides, polyurethanes, polycarbonates, fluoropolymers, silicone resins, phenol resins, maleic resins, and coumarone resins. Of these, polyesters have particularly excellent (a) chargeability, (b) fixing properties, (c) transparency, (d) gloss, and (e) resistance against the migration to vinyl chloride. Therefore, they have recently become of interest toward practical use as a binder resin.
Regarding electrostatic image developers in which the toner is admixed with fluoropolymer fine particles containing resin, various improvements have been proposed to prevent a toner-filming in which toner particles adhere to a photoreceptor during repeated development operations. For example, Patent Publication No. JP-B-51(1976)-1130 discloses a developer which includes polymer particles which are more electrically negative than sulfur in a triboelectric series. Patent Publication No. JP-B-48(1973)-8141 and Patent Publication No. JP-A-54(1979)-126031 describe developers which include polymer particles having a surface free energy lower than that of the toner, while Patent Publication No. JP-A-1(1989)-281459 describes a developer which includes powder of a low-molecular weight polytetrafluoroethylene.
Also, various improvements have been proposed in relation to methods of manufacturing electrostatic image developers in which a kneaded composite mainly consisting of a thermoplastic binder resin and a coloring pigment is finely milled, and the milled composite is mixed with fine particles which are capable of imparting properties, such as fluidity and uniform triboelectrification, necessary for electrostatic image developers, following which the powder mixture is classified.
For example, Patent Publication No. JP-B-60(1985)-4459 discloses a method for manufacturing a magnetic single-component developer in which finely milled particles of a kneaded composite including a binder resin and magnetic powder are mixed with hydrophobic silica before classification so as to improve the efficiency of the classification and eliminate adverse effects during heat treatment of the electrostatic image developer. Patent Publication No. JP-A-6(1994)-89045 describes a method for manufacturing an electrostatic image developer in which fine modifying powder is immobilized to finely milled particles of a kneaded composite including a binder resin and a coloring pigment before classification so as to provide the toner with a high fluidity and an excellent durability. Patent Publication No. JP-A-4(1992)-145449 describes a method for manufacturing an electrostatic image developer in which a fluidity-imparting agent is mixed with finely milled particles such as described above in a mixer operating at a high speed, before classification, so as to improve the efficiency of classification.
What is most important in developing latent electrostatic images using the above electrostatic image developers is the quality of images finally obtained. Therefore, developers are desired which provide an excellent image resolution and a high image density without generating fog in the non-image area or irregularity at edges of the image area, especially when developing conditions, such as the charged photoreceptor potential and the latent image potential, vary over a wide range, or especially when the performance of a developer and a development apparatus including a photoreceptor change after repeated developing operation or with the passage of time. Especially, it has been strongly desired to reduce fog caused by electrostatic adhesion of the toner to the non-image area on the photoreceptor, i.e., the unexposed portion on the photoreceptor where charge is maintained. Accordingly, studies have been conducted to reduce the fog.
To solve the above-described problems, various methods have been proposed and are publicly known. For example, Patent Publication No. JP-B-2(1990)-10419 discloses addition of inorganic fine powders having ferroelectricity, while Patent Publication No. JP-A-62(1987)-184473 describes addition of conductive fine particles. Other known methods include addition of inorganic fine powder which has been caused to have hydrophobicity by silane-containing surface treating agents, fluorine-containing surface treating agents, or titanium-containing surface treating agents. Moreover, various improvements in the manner of addition of the fine powder, charge controlling agents, and binder resins are known.
Also, Patent Publication No. JP-A-57(1982)-120943 describes a method of manufacturing an electrostatic image developer in which a toner and an additive such as colloidal silica or polytetrafluoroethylene are mixed without applying a shear force to them so as to make the angle of repose of the developer 50.degree. or smaller.
When development of latent electrostatic images was performed using a developer manufactured by those methods as described above, images having an excellent image resolution and a high image density were sometimes obtained, substantially without causing fog or irregularity at the edges of the images if the development was performed in accordance with a specific development under specific conditions.
However, latent electrostatic images developed in accordance with the nonmagnetic single-component development using the developers as described in the above-mentioned publications or the developers manufactured by the methods described therein cannot always obtain images which satisfy the requirements as described before. Some combinations of components such as binder resins, charge control agents and additives cause adverse effects, thereby generating considerable fog. Also, even in the case where the above-mentioned developers provide clear images substantially without fog, such results are generally obtained only within a narrow range of developing conditions. Accordingly, few developers have been provided which satisfy the requirements for a developer over a wide range of developing conditions (which facilitate the design of a development apparatus) even when the performance of a developer changes after repeated developing operations or a development apparatus including a photoreceptor changes with the passage of time or with the operation period thereof.